The present invention relates to a method and an apparatus for estimating a temperature contribution of an inverter used for supplying current to an electric machine, in particular a synchronous machine.
In hybrid and electric vehicles, monitoring the temperature of the power electronics plays a significant role. If the IGBTs and diodes installed in the pulse-controlled inverter, hereinafter referred to simply as the inverter, become too hot, damage to the semiconductors of the inverter and an associated failure of the electric drive may result. A temperature module is used for monitoring the temperatures. This temperature model determines the temperatures of the semiconductors and enables a timely limitation of the torque and the resulting currents.
In hybrid/electric vehicles, in which the electric drive is coupled to the wheels or to the output without a speed converter, the electric drive must therefore apply the required torque for starting or at low rotational frequencies without mechanical support. This is particularly relevant when starting on hills or steep slopes.
As a result, the inverter which is to provide this high current demand is overloaded, since, at low rotational frequencies, particularly at a rotational speed of zero, the current is applied in the worst case via the same power semiconductor of one phase for a longer period. An asymmetrical loading of the power semiconductors of the inverter occurs. Accordingly, this results in the necessary of identifying this overloading of the power semiconductors and thus reducing the current for the self-protection of the power semiconductors.
One known approach for estimating or calculating the power semiconductor temperatures is based on a linear network method. The power loss (a total of twelve partial losses) of each power semiconductor is calculated from the instantaneous currents, voltages, and the duty cycle, and a respective temperature swing is calculated from said power loss. For this purpose, the thermal dependency between the individual power semiconductors, which are embodied by IGBTs and diodes, is used. This thermal dependency is described by first-order transfer functions. For calculating the absolute maximum IGBT temperature, the maximum from the six individual IGBT temperature swings is ascertained and added to the cooling water temperature. The absolute maximum diode temperature is calculated analogously. The disadvantage of this method is its lack of feedback and the complexity of its calculation. Due to its lack of feedback, it is not robust with respect to disturbances.
In addition, it is known to use a temperature observer for calculating the mean temperatures of the IGBTs and diodes. However, the temperature observer is not suitable for temperature estimation in the case of highly transient control processes, for example, when starting or at low rotational frequencies.